High-density packaging of semiconductor devices for electronic appliances has been desired for downsizing electronic appliances. Miniaturization and thickness reduction of semiconductor parts have progressed, and packages that enable further thickness reduction and cost reduction are demanded.
Under such circumstances, techniques have been proposed to cope with thickness reduction. A plastic package proposed in JP 11-307675 A has a lead frame having exposed upper and lower surfaces. A plastic package proposed in JP 11-260989 has leads including those which are exposed and used as outer terminals.
On the other hand, efforts are being made to achieve one-chip system LSI. Two-dimensional wiring limits increasing operating speed by shortening wiring lines, and the development of techniques to achieve one-chip system LSI requires high development costs and long development time. Recently, trials have been made to achieve system LSI by using packages built up by stacking semiconductor devices for three-dimensional arrangement.
Such packages are called also system packages.
A stacked, plastic package disclosed in JP 11-307675 A is built by stacking plastic packages, and leads each having exposed upper and lower surfaces are used for electrical connection. However, those plastic packages are provided with a die pad and hence it is difficult to reduce the thickness of those plastic packages.
The plastic package mentioned in JP 11-260989 A cannot be formed in a stacked structure.
A package disclosed in JP 2002-33434 A is formed by stacking semiconductor devices (chips) and sealing the stacked semiconductor devices in a package. This package has low application flexibility and is difficult to apply to general purposes.    JP 11-307675 A    JP 11-260989 A    JP 2002-33434 A
Thus, semiconductor parts have been progressively miniaturized and made thinner, and still thinner packages have been demanded. Efforts have been made to fabricate system packages, to construct a system package by stacking a plurality of plastic packages, but further thickness reduction of the plastic packages is necessary.